(1) Field of the Invention
The present invention relates to a multipoint data packet switching communication system where the network is based upon a shared communication medium.
(2) Description of the Prior Art
Process control networks linking users can be organized into three distinct configurations which have inherent routing problems because there is no shared common communication channel:
I. Star configuration--One computer forms the center of network control (master) with separate lines to all other computers (slaves). These networks can be hierarchical since a slave computer to one star can be a master computer of a different star network. PA0 II. Multidrop configuration--Again one computer forms the center of the network control. A line is dropped to each of the other computers from a trunk line. Communication is handled by polling or is sequencing from one computer to another. Communication between computers can be accomplished only by sending messages to the polling computer. PA0 III. Ring configuration--In this configuration each computer is linked to two other computers in a closed ring. Messages from one computer to another are passed along by each intermediate computer retransmitting the message. Usually a network master exists to delete messages that have not been received.
Several difficulties arise when these configurations are applied to manufacture control systems. The networks do not adapt to problems such as system crashes, partial communication failures and recovery from complete network failures. In the star configuration, when a master computer fails, all slave computers are disconnected. This requires redundancy on all levels of the hierarchical control system. In a multidrop configuration, if a polling computer fails, no other computer can communicate. In the ring configuration any computer failure inhibits communications.
A recently introduced family of packet switched networks with decentralized control employs contention access techniques. Decentralized control eliminates a single, central controller whose failure would degrade operation of the system. The computers share one medium such as coaxial cable or a radio frequency and the contention for the medium is handled locally by each computer. The Aloha system uses a broadcast channel and transmission occurs at random. Second, slotted Aloha, a variation of the Aloha system, provides a time slot which can be shared by several computers. Transmission can only be initiated at the beginning of each time slot. Third, network systems use a carrier sense multiple access and retransmissions are based on individual time delays. A message checksum is used to detect interference caused by users transmitting simultaneously. The average packet delay is decreased by increasing the transmission rate. Ethernet handles the contention by a carrier sense multiple access (CSMA) scheme with collision detection. Transmission is permitted when idle (no carrier) is sensed. Two or more computers may sense idle simultaneously due to the propagation delay of the line. A message collision results from simultaneous transmission. When a collision is detected or a carrier is sensed, the transmitter drops off and waits a random amount of time weighted by the network traffic. Transmission is then retried. These contention networks have been used in office automation and teleprocessing applications.
A contention network allows for distributed control of communication with only passive elements used in the transmission medium. While this improves upon the reliability of previous approaches, it does not immediately adapt to manufacturing control applications. Each user of the contention network can participate in controlling communications. Furthermore, to provide high data rate service on the communication medium, all users must operate at this same high data rate unless the interface between the user and the communication medium can accommodate the different data rates. It would be desirable to improve upon the usefulness and adaptability of such a network by allowing a larger variety of user devices. In particular, it would be desirable to be able to support a relatively common user type which operates in a start-stop mode whereby information is transmitted in discrete segments. Various configurations such as star, multidrop and ring should advantageously be supported at the same time. Further, it would be desirable to solve the problem of providing an interface between a start-stop user device and a packet switched broadcast communication channel. The particular characteristics of such a user and channel must be accommodated by an interface. These problems and the problem of supporting communication connections by dynamically assigned configurations between different (heterogenous) user computer equipment are some of the difficulties this invention overcomes.